Most cities and counties have enacted requirements for industries which discharge water into the municipal sewer systems. These municipalities generally require the waste water to meet certain standards before being discharged into the sewer systems, and these standards usually relate to the level of pollutants which interfere with the efficient operation of the municipal waste water treatment facility.
Waste water which contains materials having a high biochemical oxygen demand, total suspended solids and ammonia increase the treatment cost to the city or county. The term "biochemical oxygen demand" ("BOD") refers to the quantity of oxygen utilized in the biochemical oxidation of organic matter; and the term "total suspended solids" ("TSS") refers to the total suspended solids which float on the surface of, or are suspended in, waste water and which are removable by filtering.
In some instances, a surcharge may be levied on an industrial plant that produces waste water which does not meet the standards as established by the municipality. For example, these standards may prohibit waste water containing greater than 300 mg/liter of BOD, 300 mg/liter of TSS, and/or 18 mg/liter of ammonia. Cities and counties may also prohibit industrial plants from discharging waste water having an oil and grease concentration greater than 150 mg/liter.
A city or county also has the authority to revoke an industrial plant's waste water discharge permit if that plant continues to discharge waste water which does not meet the standards set by that city or county. Therefore, an industrial plant must effectively treat its waste water to reduce the level of pollutants and thereby meet such standards.
An effective waste water treatment process for an industrial plant should cause the solid organic and inorganic matter to flocculate and form a sludge cake. This sludge cake accumulates to a certain thickness and is then separated from the liquid component of the waste water and sent to a rendering plant for further processing. If the resulting sludge cake is under 18% solids, the rendering plant may impose a surcharge because sludge with a high percentage of water is more expensive for the rendering plant to process.
One method of treating waste water from an industrial plant is disclosed in Stewart U.S. Pat. No. 4,981,599. This process results in the formation of a sludge cake on top of the waste water. The sludge cake is removed, and the treated waste water is discharged into the sewer system.
However, the sludge cake (also referred to as the float material) produced by the Stewart process is not ideally suited for subsequent treatment at a rendering plant because (1) the water content of the float material is higher than desired and (2) the float material is comprised of fatty and protein components which desirably should be separated from the water component.
Therefore, a need exists for a process which can be used to separate the protein, fatty and water components of a float material produced by a waste water treatment system. Additionally, this process should provide protein and fatty components with a high percentage of solids (i.e., a low percentage of water content).